1. Field of the Invention
The present invention relates to a rotation detector for detecting the position, speed, and the like of a rotating object.
2. Description of the Related Art
An optical pulse encoder is known as a typical conventional rotation detector. This encoder includes a disk on which rotating slits are recorded, and a pair of detectors for optically reading the rotation of the rotating slits. The rotating slits are constituted by light-transmitting and light-shielding parts which are alternately arranged on the disk at equal angular intervals in the circumferential direction. The disk is coupled to a rotating object whose rotation is to be detected (a rotating object to be detected). Each detector comprises a light source, stationary slits for detection, and a phototransistor for converting an optical signal into an electrical signal. When light-transmitting parts of the rotating slits and the stationary slits for detection coincide with each other upon rotation of the disk, light from the light source reaches the phototransistor. When light-transmitting and light-shielding parts of these slits coincide with each other, light from the light source does not reach the phototransistor. The motion of the rotating slits stored in the disk is represented by changes in intensity of light emitted from the light source and reaching the phototransistor, and is converted into an electrical signal by the phototransistor.
The stationary slits for detection are respectively shifted from the rotating slits by 1/4 the slit interval, and hence electrical signals obtained from the detectors become two-phase signal components having a phase difference of 90.degree.. Since the advance/delay relationship in phase between two-phase signal components obtained from the detectors is reversed in accordance with the rotational direction of the disk, the rotational direction can be discriminated. The above-described encoder is designed to obtain a pulse-like two-phase electrical signal whose amplitude changes once for every rotation of the disk corresponding to one slit. Hence, such an encoder is called a two-phase pulse encoder.
A great deal of attention has recently been paid to a system for driving a robot or the like by direct drive by means of a servo motor without gears instead of driving it by gear-reducing the rotation of a servo motor as in a conventional system. In this system, play, a decrease in rigidity, and the like due to gears need not be considered, and hence a high-speed operation can be performed with high precision. A servo motor used for such a direct drive operation needs to be rotated at very low speed. Therefore, rotation control for this motor requires a rotation detector having a higher resolution than a rotation detector of a motor with gears by a degree corresponding to the reduction ratio of the gears.
In an optical pulse encoder, if the slit interval becomes several tens .mu.m or less, the influences of diffraction of light cannot be neglected, and the interval between a rotating slit and a stationary slit for detection must be greatly decreased. Consequently, mechanical assembly of a rotation detector becomes very difficult (about 6,000 slits must be formed, provided that a disk having a diameter of 10 cm is used and the slit interval is set to be 50 .mu.m).
As a method of recording data at a very high density using a rotary disk, a so-called optical disk system is available, which is used for a CD (compact disk) for recording music or an LD (laser disk) for recording images. This system is designed to optically read pits recorded in a disk with a laser beam. By using this technique of optical disks, a rotation detector having a very high resolution can be obtained.
If this technique of optical disks is used, about 210,000 slits can be formed in a rotary disk having a diameter of 10 cm. A one-phase output rotation detector obtained by this technique has a resolution more than ten times that of a conventional optical pulse encoder.
Although a high-resolution rotation detector can be designed by using this technique, tracking in the radial direction of the disk must be performed to accurately focus a laser beam onto a narrow pit. This requires a mechanism for radially moving an objective lens, and hence the structure is complicated. In addition, since the rotation detector is mounted on a motor or the like, tracking may not be performed because of the influences of vibration. Therefore, it is difficult to increase the reliability of the detector.
Moreover, in order to obtain tow-phase electrical signals for discriminating a rotational direction as in a two-phase pulse encoder, a pair of photodetectors are required and must be arranged in the circumferential direction of the rotary disk at an interval of N.+-.1/4 times (N is an integer) a slit interval of 1.5 .mu.m. Since the width of each detector in the circumferential direction is at least about 10 mm, the interval between the detectors is 10 mm or more. An error of this interval in the circumferential direction must be sufficiently smaller than 1.5/4 .mu.m with respect to a dimension of 10 mm. Therefore, it is very difficult to realize a very high relative precision.